Hostel residents at IIT-Bombay "shall not participate in any anti-national...or any other undesirable activities", the institute's Dean of Student Affairs said in a mail sent yesterday. However, the letter, which also cautions students against "anti-social activities", does not specify what constitutes "anti-national" activities. The letter also comes amid a chorus of allegations by activists and opposition parties that the ruling BJP seeks to stifle critics by labelling them "anti-national"; an allegation repeated by senior BJP leader Vijay Goel this month when referring to critics of the citizenship law.